Bug: Redirect on to custom VisualForce page on click of record (Condition Based). How to fix this?

Recently come up with a situation that on clicking a record it redirects me to an custom vf page on certain condition rather than on the detail page of Salesforce record.

I have changed the view button to override on to my vf page, but the error that I'm facing is

"An internal server error has occurred
An error has occurred while processing your request. The salesforce.com support team has been notified of the problem. If you believe you have additional information that may be of help in reproducing or correcting the error, please contact Salesforce Support. Please indicate the URL of the page you were requesting, any error id shown on this page as well as any other related information. We apologize for the inconvenience.

Thank you again for your patience and assistance. And thanks for using salesforce.com!

Error ID: 178978640-125263 (-1373969722)".

Below is my code:-

public with sharing class DetailPageOnClickingRecordCls {

public id accId{get;set;}
public Account accountDetail{get;set;}
public string accName{get;set;}

public DetailPageOnClickingRecordCls(ApexPages.StandardController controller) {
}

public PageReference doTest(){

accId = ApexPages.currentPage().getParameters().get('id');
accountDetail = new Account();
accountDetail = [select id,name from Account where id =: accId];
accName=accountDetail.name;
system.debug('accName::'+accName);
system.debug('accountDetail::'+accountDetail.id);
PageReference pageRef;
if(accName.substring(0,1)=='A'){
pageRef = new PageReference('/apex/DetailPageOnClickingRecord');
pageRef.setRedirect(false);
system.debug('hello in if');
//return pageRef;
}
else{
pageRef = Page.SelectableAccountPaginationDemo ;
pageRef.setRedirect(false);
system.debug('hello in else');
}
return pageRef;
}
}